A. Field of the Invention
This invention relates to a surface controlled subsurface safety valve for controlling flow at a subsurface location in a well. The subsurface safety valve includes a main valve member. When the safety valve is open, the main valve member provides a full open bore flow path through the safety valve. Prior to movement of the main valve member to its open bore position, an equalizing valve is opened to equalize pressures across the main valve member.
B. The Prior Art
Surface controlled subsurface safety valves having both a main valve member and an equalizing valve are disclosed in United States Letters Pat. Nos. 3,583,442 and 3,703,193. For the safety valves disclosed in such patents, the equalizing valve opens prior to movement of the main valve member to its open position. If the shut-in, downhole well pressure is high, the force required to open the equalizing valve will be much greater than the force required to move the main valve member between its closed and open positions once the equalizing valve is opened. Control fluid will have to be pressurized to a pressure greater than the pressure of the shut-in well fluids to open the equalizing valve. Thereafter, that same high control fluid pressure will act to quickly move the main valve member to its open position. Such quick movement could cause damage to the valve.
An emergency condition may be created in the well if the surface well head is damaged. Under such an emergency condition, the well would flow freely due to an absence of surface controls. If surface controls are rendered inoperative and the well does begin to flow freely, subsurface safety valves will be relied upon to shut in the well. However, under such emergency conditions, with well fluids flowing freely through the subsurface safety valve, the well pressure sensed at the subsurface safety valve approaches zero. Little or no force is provided by the freely flowing well fluids to assist closure of the subsurface safety valve. The spring, or other inherent resilient urging means, is relied upon to supply the force required to close the subsurface safety valve. That spring force presently has to move a relatively large volume of control fluid out of a control pressure chamber into a control conduit. Forces, due to the hydrostatic head of control fluid within the control conduit, resist that movement of control fluid and thereby retard closure of the subsurface safety valve.